The present invention relates to a heater control device of an oxygen concentration sensor, which performs heater control of the oxygen concentration sensor provided in an exhaust passage of an internal combustion engine and having a heater for heating a sensor element. In particular, the present invention relates to a heater control device of an oxygen concentration sensor provided in an exhaust passage of an internal combustion engine capable of executing idle stop control.
Traditionally, an oxygen concentration sensor for detecting oxygen concentration or an excess air ratio to theoretical air fuel ratio (a lambda value) has been provided in an exhaust passage of an internal combustion engine which is typified by a diesel engine. Information that is detected by the oxygen concentration sensor is used for, for example, correction of variation in a fuel injection amount, correction of variation in an EGR amount (an exhaust gas circulation amount), abnormality diagnosis of an exhaust gas purifier, or the like.
As the oxygen concentration sensor, an oxygen concentration sensor is used which has a sensor element made of a solid electrolyte such as zirconia and is also provided with a heater for maintaining the sensor element at a predetermined activation temperature. Such an oxygen concentration sensor is made so as to be able to output a sensor signal according to oxygen concentration in exhaust gas when the sensor element has reached a temperature greater than or equal to the activation temperature by heating the sensor element with the heater.
Here, at the time of cold start-up of the internal combustion engine, since the inside of an exhaust pipe is in a low-temperature state, water vapor contained in the exhaust gas condenses, thereby becoming water droplets. The sensor element is heated to about 800 degrees by the heater, but if condensed water touches the sensor element, the sensor element will cool suddenly, and thus there is a concern that cracks in the element may occur due to thermal shock. For this reason, a configuration is made such that at the time of cold start-up of the internal combustion engine, control is performed so as to operate the heater after it is determined that all the condensed water in the exhaust pipe has evaporated.
As a method of determining that the condensed water in the exhaust pipe has evaporated in heater control, there is a method focused on a heat balance in the vicinity of a sensor installation site. For example, a heater control device of a gas sensor is disclosed which is made such that an ECU calculates heat amount data corresponding to a heat balance in the vicinity of a sensor installation site of an exhaust pipe on the basis of the operating states of an internal combustion engine and a vehicle after start-up of the internal combustion engine, performs determination of dryness in the exhaust pipe on the basis of the heat amount data, and controls an energization state of a heater on the basis of the result of the determination of dryness (refer to JP-A-2007-138832, for example).